Capacitor question

Hi Mark. Just the one. No it started perfectly all the time,no problem and not sluggish at all. That broken off wire was totally to blame IMHO. The last I switched it on,it tried to start,then the sparks I saw and then just hummed. I moved stuff around to clean before my opperation so I think that could have been the last straw for that wire and that's maybe why it arced a bit and then burned off.
 
Infact I think if the wire did not burn off,it would have kept going. I have this cheap old compressor since 2011 and it works every single day and still in good condition.
 
Capacitors do have a shelf life and AFAIK tend to leak after 10 to 15 years of use. You may want to source a new one on Ebay to have as a spare, as the one given to you may not have a lot of life left. My .02
 
Most of the time the starting capacitor is larger than the run capacitor. The start capacitor is what gives the motor a kickstart. Without the capacitor, the motor will just hum or vibrate. You could even tie a rope around the shaft. Pull on the rope and it will start spinning whichever the direction the rope was on. Not convenient. If the cap is too small, the motor may not get up to speed. Too big, it could overheat the motor. The capacitor is only in for short period of time to get the motor going, then it is taken out of the circuit. The run capacitor is for efficiency and it in the circuit all the time. Right size, the motor will run cool and use the least amount of current. The run cap may not be an electrolytic, but an oiled filled one stays cooler.
While I was writing this some messages came. Just found out that you have only one capacitor, that is a start capacitor. If the capacitor is 40mf and 450volt, stick with that. And buy new, capacitors are like fruit. They do have a shelf life.
 
Electrolytic capacitors can fail in one of two ways; they lose capacitance or they short out. An electrolytic capacitor is a polarized device. It has capacitance because a thin film of insulating material is electroplated on one of the electrodes. Reversing the voltage will destroy the plated coating and the capacitor shorts out. Non polarized electrolytic capacitors such as those used in a.c. induction motors are actually two electrolytic capacitors back to back they mutually protect each other from seeing a reverse voltage.

I have easily tens of thousands of electrolytic capacitors, most of which are more than thirty years old and some more than double that. Most of these have been exposed to temperature swings from -40ºF to +120ºF. It is rare that I find a bad capacitor. In fact, I can't recall the last time. If I do, it is usually caused by some abuse such as exceeding operating specs.

It is fairly easy to check an electrolytic capacitor with a simple multimeter, a resistor, and a voltage source. If a capacitor is charged to a given voltage and a resistor connected across the terminals, it will drop to 37% of that voltage when the discharge time is t = RC where t is the time in seconds, R is the resistance in megohms, and C is the capacitance in microfarads.

Connect a multimeter and a resistor to the capacitor terminals. The multimeter will present a parallel resistance and must be accounted for unless the resistor value is significantly less than the meter resistance. My multimeter has an input resistance of 9.3 megohms so a 1 megohm value for R is small enough for a decent approximation of C. Connect a 12 volt battery to the capacitor to charge it. Remove the battery and when the voltage drops to 10 volts, start a stopwatch. Stop the stopwatch when the voltage drops to 3.7 volts. The capacitance in microfarads will be the time in second divided by the resistance in megohms. In the case of 1 megohm resistance, the capacitance will equal the time. In the case of my multimeter, neglecting the meter resistance will result in the measured capacitance value being 10% low which is close enough for evaluating the condition of an electrolytic.
 
It's a compressor motor so it makes sense that it uses a smaller value cap. It's probably a permanent split-cap motor, also called a capacitor-run motor. Those often won't have an internal centrifugal start switch, and the capacitor serves as both start and run
 
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May I suggest that one read an article by www.electronicstalk.org/how-long-do-electroytic-capacitors last. Yes, you can measure capacitors with an ohmmeter. but an ohmmeter cannot measure ESR or equivalent series resistance. Or you might say very capacitor is also a resistor. In most cases the resistance is so high, it cannot be measured with normal instruments. ESR in electrolytic capacitors can be measured with the right equipment. Resistance causes heat, The higher the ESR, the more heat, Enough, the cap will blow. Aging and environment can increase the ESR. I have been in electronics for over 50 years. I think that I have replace more capacitors than all other components combined both solid state and tube.
Markba, you have a good point, I forgot about that.
 
Capacitors do have a shelf life and AFAIK tend to leak after 10 to 15 years of use. You may want to source a new one on Ebay to have as a spare, as the one given to you may not have a lot of life left. My .02
You have to allow that eBay of any effective sort is not an option for Michael (@Suzuki4evr ), he being in a country where eBay just isn't.
That said, AC motor starting capacitors are not expensive, and will certainly be available from the right store. It's OK to use used ones, These things can be very long lived. They do die from very old age if they leak, or the electrolyte dries up, or the oxide deteriorates.

How they work
Now to clear up some misconceptions about AC electrolytic capacitors. It is VERY important to only use capacitors specifically marked as AC types. This is because, values like 40uF (40 micro-Farads) can only be built in a thing that small by making the insulating gap between the capacitor plates incredibly thin. It is is, in fact, a layer of aluminium oxide that is built up by electrolysis, with a wet electrolyte.
These types of electrolytic capacitor are polarized. The DC sort can only be used connected the correct way around. You see them in any electronic circuit.

So what makes a "AC" motor capacitor?
Hidden inside the can is a construction that is actually two capacitors connected in series, with one reversed, so to have polarity connected "back-to back". In normal use, a DC electrolytic capacitor always has the voltage replenishing and preserving the oxide layer. If connected the wrong way, the leakage current is dissolving the oxide, and eventually it breaks down, and the capacitor blows up. I have had this happen on a tiny 100uF capacitor. It was loud, and it made a huge mess!

With AC connected capacitors, in series, the actual capacitors in there are double the value of the final series combination.
2 of 80uF in series becomes only half that, i.e. 40uF total, but the benefit is the voltage breakdown rating is doubled.
With AC 50Hz or 60Hz current, at any time in the cycle, one of the series pair is the "correct" way round, there is not time for the other to have it's oxide break down. For 50Hz, only 20mS cycle time, so only 10mS being "reversed".

When there are two capacitors in the can.
It's actually four, but connected internally in series, so you only see two.

What if there are three connections?
Some capacitor connections to motors use the outer of the can, insulated, to be the common connection used by two capacitors, such as might be for "starting" and "running" capacitors. More likely, if not insulated, the outer (bare) can is to a mounting bolt. This would still be to ungrounded parts of the motor wiring. Otherwise, its to a frame safety ground. This is stuff one needs to take care over.

"Micro" Farads?
Yes - a millionth of a Farad. (named after Michael Faraday)
A whole Farad is a capacitor that would get up to only 1 Volt, even from running a current of a 1 whole Ampere into it for 1 whole second.
It is, in fact, a large amount of energy. That is what you call a "supercapacitor", as used in Formula 1 cars.
40uF at 450V is a decent size, small enough physically. It gets up to hundreds of volts in a quarter of a cycle, so 5 milliseconds in RSA, or 4.16mS if in the USA, where mains is 60Hz

Why with a motor?
This is to be able to drive single phase motors. The coils in the motor can make a magnet field between them, but they need another set of coils made rotated 90 degrees, to continue to pull on the rotor when it has turned some. You can connect the mains live power to one coil set, but you need another mains power running out of phase by 90 degrees relative to the first, to connect to the second set of coils.

The way the "second" power is derived is to put it through a AC capacitor. It initially charges up, and then, when in series with the motor coil, makes the "other" AC power that is phase shifted as needed. The motor will turn, and in the correct direction.

I know all this is a bit of a rough, qualitative explanation, but I hope it helps. Know that 5% is quite accurate. The motor will likely still work OK if the capacitor value is even 20% wrong, but it helps if the value is "higher" rather than lower. Also, most motors do not have a special separate starting capacitor. One often sees capacitors in motors with one connection not used. Note also the high voltage rating. Do not use capacitors in mains motor circuits with ratings less than the 450V norm.
 
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You need to replace it with the same type of capacitor that is used, you cannot always use an electrolytic capacitor to replace another type of capacitors. Typically AC electrolytic capacitors are of high uF for their size, and and are used as start capacitors. Leave them in the circuit for more than a second or two and they will go up in smoke quickly due to overheating. Run capacitors which are left in the motor circuit are typically solid wound type like metalized polypropylene or oil filled capacitors, they are much larger in size for the same capacitance as an electrolytic. Electrolytic capacitors do have a shelf life, there is a chemical reaction and they degrade over time. They may test OK for uF, but apply voltage to them and they can quickly go south. Older electrolytic capacitors it is recommended that if used, that the voltage be applied in stepped stages to reverse some of the chemical degradation process. or reforming. Almost every VFD manufacture has cautionary warning about the shelf life of VFD (electrolytic) capacitor's that are not powered up annually. That being said, it depends on the type of electrolytic capacitor, the manufacturing, the voltage/application, etc.

If you look up the specification for the capacitor in your compressor you will see it listed as metallized polypropylene film, so it is most likely a run capacitor. Some motors will use the same capacitor for the start circuit, it is always energized. You do not want to replace it with an electrolytic, You need to replace it with a 40uf, 450VAC capacitor of the same type and rating. Electrolytic capacitors usually have a wide range as to their uF ratings so something like +/-20% of the value or more, other types of capacitors are usually much tighter as to their capacitance range.
 
You have to allow that eBay of any effective sort is not an option for Michael (@Suzuki4evr ), he being in a country where eBay just isn't.
Yes Graham you are right that there is not an eBay here but I can buy on eBay from China,the UK and such,even the US,but that is just not financially sensable if you keep shipping in mind. Shipping from the US to SA is rediculas . I can buy a new one localy for R240(around $14), and I know it is not that much, but at some point I must just think of not over spending on a reasonably old machine,because I have spent some money on it in the past. So knowing that I am not going to buy a new compressor now,I will take the second hand cap for now because it was free....if it works well then it works,if not,then I will buy the new cap and be done with it.


Yes - a millionth of a Farad. (named after Michael Faraday)
They could not have chosen a better name :big grin:

Thanks to everybody sofar for your input and the great and detailed information. I am saying I understand it all,but I think I learned a little about capacitors.

Michael
 
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